In conventional freezing tunnels liquid nitrogen is admitted to the freezing tunnel through a plurality of spray nozzles. The flow of liquid nitrogen to the spray nozzles is continuously varied by a flow control valve in response to the difference in temperature at a point in the freezing tunnel and a set temperature. In a typical installation, after initial cooldown, the flow through each spray nozzle is varied from 0 l/s to 1.4.times.10.sup.-2 l/s which corresponds to a pressure immediately upstream of the spray nozzle (i.e. downstream of the flow control valve) of from 0 to 1 bar (gauge).
We have discovered that the heat transfer coefficient for any given spray nozzle varies with the flow through the spray nozzle and that, in many cases, the heat transfer coefficient is particularly high over a very narrow range of flow. Since flow is a function of the pressure applied upstream of the spray nozzle it therefore follows that the heat transfer coefficient for any given spray nozzles varies with the pressure immediately upstream of the spray nozzle. In addition, we have discovered that the effectiveness of any given spray nozzle also varies with the pressure immediately upstream of the spray nozzle.
In general, the higher the heat transfer coefficient the quicker a product will be frozen. Similarly, the higher the effectiveness the greater will be the proportion of the available refrigeration which will be applied to the product to be frozen.